The Vega homeworld is becoming unstable due to the exploiting of Vegatron, a powerful radioactive ore. Seeking to expand his militaristic empire and find a substitute planet to settle upon, the ruthless King Vega unleashes his armies - composed of flying saucers and giant robotic monsters - and turns first against neighbors such as Fleed, a highly advanced but peaceful world. In a tragically ironic twist, the invaders' blitzkrieg turns against them: the once verdant, idyllic Fleed is turned into a radioactive wasteland. Too late, the only known survivor of the royal family, Prince Duke Fleed, manages to steal the Grendizer, the robotic embodiment of the Fleedian God of War, from the Vegan invaders who plan to use it to spearhead their invasion fleet.

Fleeing Vegan space by flying at faster than light speed, Duke enters the Solar system and switches course to Earth, making a rough landing in Japan, on the slopes of Mount Fuji ...